Channel-tolerant FH-MFSK acoustic signaling for undersea communications and networks

Undersea acoustic channels can exhibit multipath propagation with impulse-response duration and coherence time both of the order of tens to hundreds of milliseconds. Signal reception is further impaired by the presence of time-varying nonwhite ambient-noise spectra having a dynamic range of 30 dB or more. Acoustic communication requires appropriate waveform design and associated signal processing to accommodate these adverse transmission characteristics while also providing desired performance features such as low-probability-of-detection (LPD) and multi-access networking. Adaptive-equalization techniques provide good performance only for channels with stable multi-paths and high signal-to-noise ratios (SNR\´s) accommodating the signaling rates needed to sample and compensate for rapid changes. An alternative approach is to design for robustness against channel fluctuations. This paper describes a channel-tolerant approach identified as "telesonar type-B signaling." The technique has been designed to accommodate network architectures requiring multiple access to the channel while simultaneously providing covertness and energy efficiency. Specialized frequency-hopped M-ary frequency-shift-key (FH-MFSK) waveforms are combined with related signal processing, including nonlinear adaptive techniques to mitigate the effects of all types of interference. This effectively results in a channel that has uniformly distributed noise in both time and frequency. Powerful error-correction coding permits low SNR transmissions. Nonbinary, long-constraint-length, convolutional coding and related sequential decoding is a classical solution for difficult low-rate channels. The probability of bit errors below 10/sup -10/ is obtainable, even in Rayleigh-faded channels near the computational cutoff rate, and the probability of failure to decode frames of data is extremely small. Both simulations and analyses of at-sea experiments demonstrate the performance of this noncoherent approach t- reliable acoustic communications.